Method of fixing sulphur dioxide as sulphuric acid and ferric sulphate



Sept. y22, 1936. H. E. KEYES 2,055,082 METHOD OF FIXING SULPHUR DIOXIDES SULPHURIC ACID AND FERRIG SULPHATE original File sept. 4. 19'31INVENTOR` )Ya/mw. i ffeys Y v ATTORNEYS 'Patented Sept. 22, *19316,*

Mmnon ornxmo s sou'nmnc -Aoro nai-mon n. Keyes,

Application September 4. 1931, Serial No. 561,159

-- i Renewed March 14, 1933 ULPHUR'DIGXIDE AS AND FERRIC SUL- *Rassen 12o. (el. 2x2- 167) This invention relates to an improved method i offixing sulphur doxidein the formof sulphates, and, more particularly, tothe fixation of sulphur dioxide lin the form of sulphuric acid 5 orferrie sulphate or mixtures of these sulphates by an improved methodinvolving treating solutions containing ferrous sulphate with sulphurdioxide and air. Any, or all, ofthe following reactions may take placewhen solutions of.` ferrous sulphate are treated with sulphur dioxide'and air, depending on the factors controlling gas absorption and therelative concentration of sulphur dioxide, air, ferrous sulphate andsulphuric acid: v

It has been proposed heretofore to introduce mixtures of sulphur dioxideand air into ferrous sulphate solutions by mechanically mixing thesolution'with the gases or by passing the gases' through a porous mediumor other small orices into the solution. Since sulphur dioxide gas maybeoxidized or may act as a reducing agent itself, the prior processes havealways been limited to relatively low percentages of sulphur dioxide inthe air-sulphur dioxide mixture. In

fact, if the percentage of sulphur dioxide exy 3o ceeds approximately 4%in the mixture, the

iron is reduced faster than it is oxidized, according to reaction (c).If the iron becomes entirely reduced, this reaction ceases and it isnecessary to use a large excess ofV air to reoxidize the 35 iron andallow thevreaction to proceed. Thesel absorbingrelation to the ferroussulphate solution under conditions whereby the sulphur` di- 45- oxide orSO2-containing gases exist substantially as a continuous phase oratmosphere and the ferrous sulphate solution exists as a discontinuousphase; that is, it is present in more or less dispersed or disseminatedform in the at- ,50 mosphere of SO2-containing `gases during the4absorption stage. Air or other oxidizing gas under pressure is thenbrought into absorbing rrelation to the resultant solution, preferablyby0 introducing the air into a body of the solution re- 55 sulting from.the gas-absorption step previously mentioned, and these operations arerepeated in the order named until substantially all of the ferrous ironhas been oxidized to the ferrie state and the sulphuric .acid content ofthe solution has been built up to the desired extent. e:

- I have found that by thus repeatedly bringing the liquid intoabsorbing relation to sulphur dioxide and then into absorbing relationto air, the .rate of acid formation is increased and the acid content ofthe solution may be built up to a 10 much greater extent than intheprior processes.

One of the advantages of my invention is the utilization of impuresulphurous gases, such as roaster gases, containing 1% to 14% of sulphurdioxide and varying amounts of sulphur trioxide, l5 oxygen, carbondioxide, nitrogen and nue dust. It is further possible to use thisimpure gas with- *out pretreatment of any kind. Such gases do not lendthemselves readily to treatment by methods of SO2-gas absorptioninvolving injection of the 20 gas beneath the surface of a body of theabsorbing solution. In the first place, in order to secure adequate SO;absorption with a dilute SO2-con-` taining gas, it is necessary toinject or blow the gas into the solution under a head of several feet,and this denitely requires gas-pump blowers o the positive pressure typewhich operate with close clearances. Owing to the necessltyof handlinglarge volumes of gases, a preponderating part of which may be inert ortake no useful part in the process, the power costs of injecting thegases under the pressures required are considerable. Secondly, whendealing with impure gases such as roaster gases diiliculties such ascorrosion and sticking, arise due to the corrosive nature of the gasesand solid impurities contained therein. My invention has the advantagethat the impure gases may be introduced at substantially atmosphericpressure, their movement to and through the absorption zone, beingconven- 4o iently effected by the use of Ventilating type blowers whichare `cheap as compared with gas pumps and operate with such. largeclearances that dust and soot cannot interfere. Furthermore, such fanscan be readily acid proofed as by lead or rubberlining, something thatis not feasible in the case of positive, pressure gas pumps.Stillfurther, when operating according to the principles of myinvention, the desired circulation of the SO2-containing gases may beeected by i`n- 50 troducing the fan in the path of ow of the gases at apoint beyond the absorption chamber, and hence after the gases have beenlargely or substantially completely stripped ofthe sulphur dioxideconstituent as well as of solid impurities.

My process may conveniently be carried out in the apparatus shown in theaccompanying drawing. This apparatus is described and claimed in myco-pending application Serial No. 561,160 (now U. S. Patent No.1,952,675). It will be understood that the process may be carried outwith the use of various types of apparatus other than 'that disclosed.

In the drawing: Y

Fig; 1 is a plan of the apparatus with the cover removed. i

Fig. 2 is a sectional elevation taken on the line 2-2 ofFig. 1. Y

Fig. 3 is a sectional elevation taken on the line 34-3 of Fig. 2.

In its essential features, the regenerator consists of a container I0,having vertical partitionsA I2, which form one or more air lifts I 4. Ahorlzontal grating I6 is provided with slots I8. The container IB may beprovided with a cover 38 having vents 40.

ln operation, the ferrous sulphate solution is fed into the regeneratorcell through the pipes 2li and the solution resulting from the treatmentleaves through the pipes 22, thus maintaining the liquid level as at 24.Air under sumcient pressure to overcomethe hydrostatic head of the liqbythis means and overflows onto the grating I6 and passes through theslots I8, whence it passes in the form of small streams, sheets orcascades through the gas lled space 32. These streams, sheets orcascades impinge on to the surface 2t of the main body of the liquid.Openings 42 from the main body of the liquid in the regenerator to theair lifts are provided so that the liquid may be circulated repeatedlythrough the air lifts I4 and the gas chamber 32 before passing out ofthe cell at 22. 'I'he gas containing sulphur dioxide is introducedthrough pipes 34 and is Vdrawn. out of the cell through pipes 36 bymeans of a suction fan not shown. The sulphur dioxide gas is thusbrought into intimate contact with many streams of liquid'fallingthrough the spaceii2 vonto the surface of the liquid.

In an actual operation using this type of apparatus, the sulphur dioxideges contained 10% sulphur dioxide and the solution fed to the apparatuscontained'lO lbs. per ton of iron in the ferrous condition. Air undersuper-atmospheric pressure was introduced into the air lifts in amountsuilicient for rapid circulation of the lids uid. After six hourstreatment, 98% of the iron was oxidized to the ferric condition and 67lhs. of sulphuric acid per ton of solution was produced.

By properly adjusting thevolume of sulphur dioxide 'gas to the volume ofsolution passing through the apparatus, it is possible to utilize overof the sulphur dioxideiin the production of ferric sulphate andsulphuric acid.

By employing a countercurrent ow of sold?- tion and gas advance in acontinuously operated cell, the outgoing spent gas makes contact withthe incoming ferrous sulphate solution, thereby resulting in a highdegree of absorption of sulphur dioxide. At the other end of the cell,the outflowing ferric sulphate-sulphuric acid solution makes contactwith the strong incoming sulphur dioxide gas. If it is desired toproduce only sulphuric acid, the ferric sulphate may be entirely reducedby introducing a highly concentrated sulphur dioxide gas, or a greaterproportion of gasto solution -into the outgoing solution. This may beaccomplished in the regenert ator cell proper or in 'a separateabsorption system into which the strong gas and the outgoing vsolutionare introduced countercurrently and the exit gas from the absorptionsystem is in troduced into the regenerator cell.- This reaction (c) notonly reduces the iron, but produces sulphuric acid.

It will be understood that in the normal operation of the processstarting with a ferrous sulphate solution, the ferrous sulphate is firstconverted-to ferric sulphate, sometimes practically completely beforemuch acid is formed.`

` and in many cases there is hardly any trace oi' ferrous iron in thesolution after the initial oxidation. Whether this may be explained onthe ground that the reaction producing sulphuric acid is catalytic withrespect to ferric iron or on other grounds, I am not prepared 'to statedenitely. Because of this fact, it' will be understood that it is notessential when aiming to iix sulphur dioxide as sulphuric acid'to startwith a ferrous sulphate solution, or to carry on the v oxidation of theferrous sulphate in a solution as a. part oi the same operation.Obviously, when so operating the process, it maybe applied to ferricsulphate solutions derived from any source or by the separatepreliminary conversion of ferrous sulphateto the 'ferric state by meanssteps, orA otherwise. It has been proposed heretofore to use ferricsulphate solutions when aim- A ing to x sulphur dioxide as sulphuricacid, see Bureau of Mines Bulletin 260, pages 19-21, and. therefore, Iam not making any claim to novelty of the use of such a solution exceptwhen used in the .manner recited in this specification and the appendedclaims. The term iron sulphate is used in-this specification and theappended claims in 'the general sense to mean ferrous sulphate andferric sulphate or mixtures of these salts.

Among the advantages of my method overl former methods are thefollowing: Sulphurdioxide of any strength may be used at atmos phericpressure and because of the stronger sulphur dioxide gas that it ispossible to use. the

rate of acid formation is considerably greater' than in previousmethods; the alternate absorption of sulphur dioxide in the solution andthe oxidation of the sulphur dioxide in the presence of iron sulphateincreases the rate of acid formation; the percentage of absorption ofsulphur dioxide gas is very high; the process is moreA stable chemicallyand more easily controlled am it is not subject to frequent alternatelyoxidizing and reducing conditions, such as were present in othermethods; in a continuously operated regeneration apparatus,countercurrent now of gas and solution advance is possible, therebyproducing high emciency of conversion of sulphur 65 `dioxide intosulphuric acid and ferric sulphate;

'sure" as used herein is to beunderstoodas including those smalldeviations from the true atmospheric pressure that may be required toeffect circulation of v they SO2-containing gases through the gasabsorption chamber. Generally a plus or minus vpressuremof a few incheswillsuice to effect the desired ow thus permitting the use ofVentilating fans and similar low pressure type blowers. The pressure maycloselyv approach the true atmospheric if theabsorption chamber isappropriately designed. o When dealing with impure SO2-containing gasessuch' as roaster gases, the maximum-advantages from the economicstandpoint are de rived by maintaining the gasesin the gas absorptionstep at substantially atmospheric pressure.

"However, it will be understood that my invention is not limited tooperationscarried onwith the gases at any particular pressure.Furthermore,

the process is applicable to thetreatment of SO2-containing gases of anydegree of SO2 concentration, including pure gaseous SO2.

It will `be understood that various changes may be made in the detailsof the procedures involved without departing from. the invention, whichis not to be deemed as limited other than as set forth in the appendedclaims.

I claim:

1. The method of producing a solution con'- taining ferric sulphateand'sulphuric acid which comprises passing a ferrous sulphate-containingsolution in disseminated form through an atmosphere of sulphurdioxide-containing gases 4under pressure conditions in such atmospheresubstantially the same as those of,atmospheric air, thereby abso-rbingsulphur dioxide into the solution; collecting the solution and passingair through said solution in an amount sufficient' to insure the xationof-l the sulphur `dioxide absorbed in such solution n the form. offerrie sulphate and sulphuric acid, and re'p`eating the statedoperations until the desired concentration of sulphuric acid and ferriesulphate is built up.

"2. The method of producing a solution containing ferric sulphate andsulphuric acid which comprises passing sulphur dioxide-containing gas asa continuous phase and under pressure conditions approximating those ofatmospheric air through an absorption 'zone while passing a ferroussulphate-containing solution through said zone as adiscontinuousphase,thereby absorbing sulphur dioxide into the solution;collecting the solution into a body; injecting air into saidbody ofsolution, and recirculating the solution between said body and said zoneuntil the desired concentration of sulphuric acid and y niospheresubstantially the same asjthose of atmospheric air,'thereby absorbingsulphur dioxide into; the solution; collectingi the solution and passingair through said solution in an' amount suicient to insure the fixationofthe sulphur dioxide absorbed in said solution in the form of ferricsulphate and sulphuric acid, and repeating the stated operations untilthe desired concentration ofsulphuric acid and ferric sulphate is builtup.

4. The method of producing a solution containing ferric sulphate andsulphuric acid which comprises establishing abody of solution containingferrous sulphate, passing a stream of gases containing sulphur dioxidethrough a space above and in free communication with said body ofsolution, raising a portion of said 'solution above the normal .level4thereof and causing'it to fall in small streams through said gas spaceinto said body ofsolution to absorbl sulphur dioxide,

and injecting air into said body of solution under super-atmosphericpressureand repeating the stated operations untilv the desiredconcentration of sulphuric acid and ferric sulphate is developed in saidsolution.

5. The process of producing taining ferric sulphate and sulphuric acidwhich comprises establishing a body of solution containing` ferroussulphate, passing a stream of gas containing sulphur dioxide through aspace above and in free communication withsaid body of solution,injecting air under super-atmospheric pressure into a partially isolatedportion of said body of solution and thereby causing a portion of saidsolution to be lifted above the normal level of' said body, then causingsaid lifted solution to cascade through said gas space into said body ofsolution to absorb sulphur dioxide and repeating the stated operationsuntil the desired concentration of sulphuric acid and ferric sulphate isdeveloped in said solution.

6. The method of iixing SO2 as sulphuric acid which'comprisesdisseminating a solution containing iron in sulphate form in anatmosphere of sulphur dioxide-containing gases under pressure conditionsin said atmosphere substantially the same as those of atmospheric air,thereby absorbing sulphur dioxide into the solution; collecting thesolution, bringing air under super atmospheric pressure into absorbingrelation to said solution in an amount suiilcient to insure fixation ofthe sulphur dioxide, and repeating the stated operations until thedesired concentration of sulphuric acidis built up.

7. In a method of producing sulphuric acid by absorbing sulphur dioxideand oxygen in ferrous sulphate solutions, the steps which comprise esitablishing a body of solution containing ferrous sulphate, passing astream of gases containing sulphur dioxide through a space above and infree communication with said body of solution, raising a portion of saidsolution above the normal level thereof and causing Iit to fall incascade lformation into said body of solution to absorb sulphur dioxidefrom said gases, injecting air under super-atmospheric pressure intosaid solution in an amount suflicient toinsure the xation of the sulphurdioxide absorbed in said solu'- tion, repeating the statedoperations-until the .iron content of said solution has been `substan-Atially converted to ferrie sulphate, and iinally reacting the ferricsulphate to ferrous sulphate and sulphuric acid'by treating the solutionwith gases containing a high percentage of sulphur dioxideas comparedwith the sulphur dioxidecontaining gases used in the earlier stage ofthe process.

a solution con- 8. In a method olf-producing sulphuric acid by absorbingsulphur dioxide and oxygen in ferrous sulphate solutions, (the stepswhich comprise establishing a body of solution containing ferroussulphate, passing a stream ofV gases containing lsulphur dioxidethrougha space above and in free communication with said body ofsolution, raising a portion of said solution above the normal levelthereof and causing it to fall in cascade formation into said body ofsolution to absorb sulphur dioxide from said gases, then'bringing saidsolution in gas absorbing relation to air under super-atmosphericpressure, repeatingA the which comprises passing a ferroussulphate-containing solution in disseminated form through an atmosphereof gases containing sulphur dioxide under pressure conditions in saidatmosphere 4substantially the same as those of. atmospheric air, therebyabsorbing sulphur dioxide into the solution; bringing anoxygen-containing gas under super atmospheric pressure into absorbingrelation to the resulting solution, thereby converting the iron contentof said solution to ferrie sulphate; andv finally reacting the ferricsulplate to ferrous sulphate and sulphuric acid by treating the solutionwith gases containing a 40 relatively high percentage of sulphur dioxideas compared with the gases used in the rst step of the process. Y

10. The method of xing SO: as sulphuric acid and ferrie sulphate whichcomprises passing a ferrous sulphate-containing solution in cascadeformation through a, zone containing an atmosphereof SO2-containinglgases into a body of said solution, thereby absorbing SO2 into thesolution, passing air through said body of solution to x the SO2absorbed therein in the for/ri of ferrie sulphate and sulphuric acid,and

circulating the solution between said body and. y

said zone until the desired concentration of sul phuric acid and ferriesulphate is built up.

11. The method of xing SO2 as sulphurlc acid and ferrie sulphate whichcomprises passing a ferrous sulphate-containing solution in dispersedform through a zone containing anl atmosphere of SO2-containing gases,thereby ab' sorbing SO2 into the solution, collecting the solution intoa body, injecting air into said body of solution to fix the S02 absorbedtherein and to induce recirculation of portions of said solution betweensaid body and said zone, and repeating the-stated operations untily thedesired concentration of sulphuric acid and. ferric sulphate is builtup.

12. -The method of fixing SO2 as sulphur-ic acid which comprises passingan iron :sulphate-containing solution in dispersed form through a zonecontaining an atmosphere of sulphur dioxidecontaining gases into a bodyof said solution adjacent thereto, thereby absorbing the SO2 into thesolution; bringing air under super atmospheric pressure into absorbingrelation to said solution to fix the S02 absorbed in such solution, andrepeating the stated operations until the desired concentration ofsulphuric acid is built up.l g

AIHIARJMON E. KEYES.

